On this day in 2017 at 1:57 UTC, Segregated Witness activated on the Bitcoin mainnet at block height 481,824. The upgrade, defined in BIP 141, fixed transaction malleability, increased effective block capacity, and laid the groundwork for the Lightning Network. SegWit activation was the hard-won culmination of the blocksize war -- a multi-year battle that proved Bitcoin's consensus rules are controlled by node-running users, not miners or corporations. The first SegWit transaction was mined in the very same block.
Bitcoin Wiki: Segregated Witness